Aluminum-containing diffusion coating for metals



United States Patent 3,220,876 ALUMHJUM-CGNTAINING DIFFUSION COATING FORMETALS Roger D. Moelier, Simi, Califl, assignor to North AmericanAviation, Inc.

No Drawing. Filed June 24, 1964, Ser. No. 377,507 13 Claims. (Cl.117-114) This invention relates to a method of providing analuminum-containing diffusion coating on a base metal or alloy, and moreparticularly to a method for providing a codeposited aluminum-silicondiffusion coating on a ferrous metal.

This application is a continuation-in-part of my copending applicationSerial No. 85,457, filed January 30, 1961, and since abandoned.

Industrial and commercial uses of metals at high temperatures and incorrosive environments have continually increased, and operationsrequiring metals with improved special properties are steadily rising.For a great many uses, it is essential that metal parts resist oxidationand other chemical surface reactions at high temperatures and abnormalconditions. Further, it is frequently desirable to have metal alloys ofvariable compositions, for example, a very hard, corrosion-resistantsurface, and a base having good Working characteristicsproperties whichfrequently are not found with an alloy of uniform composition. Metalshaving corrosion-resistant surfaces at high temperatures are required,for example, for the following items: furnace parts, turbine blades,petroleum refinery equipment, jet engine sheet metal work, superheatertubes, and chemical plant equipment; the list of applications forferrous metals with corrosion-resistant surfaces is virtually limitless.Thus, a process which can efficiently improve the temperature corrosionproperties of metals, particularly ferrous metals, is 'of considerableinterest.

The resistance of metals to surface attack on exposure to elevatedtemperatures depends largely upon the type of scale formed. The physicalproperties of the film, such as its adherence to the base metal, itsporosity, and its tendency to crack, determine the ability of the metalto withstand surface attack. The type of scale formed depends uponchemical properties, such as its composition, density, melting andboiling points.

Diffusion coatings on metals and alloys have been used to change one ormore metallurgical properties, for example, to produce an atmosphericoxidation-resistant surface, a surface resistant to the action ofspecific chemicals, and a Wear and oxidation-resistant surface. It maybe convenient to first make a particular piece from an alloy which is ineasily formable condition, and then add the diffusion layer coating toobtain the desired property. A diffusion layer, which usually involvesformation of an intermetallic compound or a solid solution of thecoating metal with the base metal, has distinct advantages over a simpleplating layer.

Although a few methods have been developed for the coating of a basemetal with a protective layer of aluminum, each of these leaves much tobe desired. A Widely used commercial procedure involves cleaning thesurface, applying flux, and then hot dipping in molten aluminum. Themajor problem appears to be the attainment of suitable adherence of analuminum coat to the base metal.

An object of the present invention is to provide a method of forming analuminum-containing diffusion coating on base metals and alloys.

Another object is to provide a method wherein the diffusion material maybe applied to all surfaces of a base metal independent of theconfiguration or shape of the base metal.

Another object is to provide a method of forming an alloy havingdifferent surface and body characteristics, thereby providing an alloyhaving surface characteristics of improved hardness and corrosionresistance.

Still another object is to provide a non-electrolytic method of applyinga protective aluminum-silicon diffusion layer on base metals,particularly ferrous metals.

A further object is to provide an aluminum-containing surface diffusioncoating of a graded alloy of variable composition on a ferrous metal inorder to increase its hardness and resistance to high temperaturesurface corrosion.

In accordance with the present invention, a base metal is provided withan aluminum-containing diffusion coating by placing the base metal in amolten sodium bath containing at least aluminum dissolved therein, andthen maintaining the base metal in the bath for a sufficient time forthe dissolved aluminum to diffuse into the base metal to form adiffusion coating containing aluminum on the base metal.

In certain preferred aspects of practicing the invention, both aluminumand silicon are dissolved in the molten sodium bath, while maintainingan inert atmosphere, and both are diffused into the base metal to form acodeposited diffusion coating therewith.

It is an essential feature of this invention that the diffusion coatingcontain at least aluminum, although it is contemplated that othercoating materials may be codiffused and codeposited therewith,particularly silicon. Optionally, other additives may be present in thebath so as to enhance the quality of the diffused coating or shorten thetime required.

There will usually be diffusion of the coating material into the basemetal, with resulting formation of a solid solution, alloy, orintermetallic compound. Diffusion layers with graded boundary layersvarying from 0.1 mil up to 60 mils may be obtained, although diffusioncoatings typically varying from 0.5 mil to 10 mils are preferred. Thediffusion of the coating material into the base metal accounts for thechanged metallurgical properties of the resulting article, for instanceimproved mechanical and chemical properties. The resulting diffusionlayer has a number of distinct advantages over a simple plating layer;an atomic bond is formed, and the surface is graded in composition.There is no sharp interface, which is beneficial in preventing spallingor breaking of the coating.

While the mechanism of diffusion transport of the dissolved aluminum oraluminum-silicon into the base metal and the formation and nature of thealloy diffusion layer (e.g., intermetallic compound or solid solution)is not fully understood, it appears that the sodium metal is largelyresponsible for the success of the process. The molten sodium providesan excellent environment for maintaining the base metal clean of oxideand organic films, not only initially but throughout the coatingoperation. Further, the sodium provides the vehicle for diffusionmaterial transport and maintains a constant supply of aluminum oraluminum-silicon at the base metal surface.

The present process may be used for applying a diffusion coatingcontaining aluminum on a large number of base metals, singly and incombination, and there are no limitations as to the base metals, exceptas may be hereinafter indicated. For example, ferrous metals may becoated with aluminum or with a co-coating such as one ofaluminum-silicon. While diffusion coatings of aluminum per se oraluminum-silicon are particularly preferred, other materials may becodeposited with aluminum, so that coatings consisting ofaluminum-nickel, aluminum-niobium, aluminum-zinc,aluminum-nickelniobium, may be obtained. Since these coating materialsdiffuse into or react with the base metal, highly complex protectivelayers or coatings are formed on the base metal, which may consist ofbinary, ternary, quaternary, and even more complex compounds.

Sodium, which includes alloys thereof such as sodiumpotassium (NaK), isthe molten carrier used in the practice of this invention. An inertenvironment, i.e., a nonoxidizing one, such as may be obtained with avacuum or by maintaining an inert gaseous atmosphere provided by noblegases such as helium, is maintained over the molten sodium. Thediffusion rate into the base metal and the solubility in the sodium ofthe aluminum and other metals codeposited therewith are functions oftemperature, and it is accordingly desirable to maintain the temperatureof the molten sodium bath as high as possible. The maximum temperatureis limited by the boiling point of the sodium or alloy thereof at agiven pressure. A satisfactory temperature range for a sodium bath isabout 1000- 1500 F.

The desired concentration of the aluminum and of the codepositeddiffusion coating materials in the sodium bath is essentiallyself-controlled by their solubility in the bath. This may vary from arange of about to 1000 parts per million, and may reach several percent,being markedly dependent upon the temperature of the bath and theproperties of the solute. Generally, with respect to aluminum, it ispreferred to maintain a small reservoir or pool of molten aluminum inthe molten sodium bath so as to insure saturation conditions of thealuminum in the bath.

The aluminum and codeposited materials used to form the diffusioncoating can be dissolved in the molten sodium bath in any convenientmanner. Preferably, the aluminum is introduced as a pure metal in eitherpowdered or molten form. Where codeposited coatings are used inconjunction with aluminum, they may be introduced as the pure metals perse in either powder or molten form, for example tin, zinc, or silicon ora molten alloy such as aluminum-nickel or other aluminum alloy may bedissolved as such to provide the codeposited coating with the aluminum.

The base metal may be any metal which does not dissolve in molten sodiumat an appreciable rate for the time of immersion used. Since sodium isnon-reactive with most of the common base metals, the selection of basemetal is relatively broad. Of particular interest are the ferrous metalsin view of their wide usage and nickel and cobalt base compositions(i.e. the first triad of group VIII). Also of interest are group IVBmetals such as titanium, group VB metals such as niobium, and group VIBmetals such as chromium, molybdenum, and tungsten. Reference made hereinto the various groups of elements refers to the Periodic Chart of theElements as shown in Handbook of Chemistry, 10th edition, N. A. Lange,editor, McGraw-I-Iill Book Company, New York, 1961, pages 56-57.

The factor which controls the time required for the diffusion coating isthe rate of diffusion of the coating metal into the base metal. Thediffusion rate is dependent upon such factors as the temperature of thebath, the concentration of coating material in the bath, the metallicstructure of the base metal, and circulation of the bath constituents.

Where the sodium bath is held at a temperature approaching its effectivelower limit of 1000 F., the diffusion rate is relatively slow anddiffusion times of 50 hours or more may be required to give a diffusionlayer of several mils on the base metal. Generally, when too long a timeis used for the diffusion coating, there is a tendency for the coatingto become brittle. Higher bath temperatures resulting in shorter coatingtimes are therefore preferred for most applications. Since the boilingpoint of sodium is about 1600 F. at which temperature the sodiumevaporates at a rapid rate, a temperature of 1500 F. is generallypreferred as a maximum working temperature. While a diffusion coating of0.1 mil may be obtained in as short a time as a minute, coatings of 0.5mil to 10 mils are preferred for protecting the base metal. Thickercoatings will be used where the coating is subject to a degradativeattack in addition to functioning as a protective coating for theunderlying base metal. Since approximately 1.5 weight percent aluminumdissolves in molten sodium under optimum conditions, baths containingfrom 0.5 weight percent aluminum to 4 percent aluminum, based on theweight of sodium, are used for diffusion coating with aluminum alone.Where silicon is codeposited with the aluminum, amounts of siliconvarying from 2 to 20 weight percent, based on the weight of the sodium,are added to the molten sodium bath.

Because of the industrial importance of providing a protective coatingfor a ferrous material, such as mild steel (AISI 1008 steel),codeposited coatings of aluminum and silicon on mild steel represent apreferred feature of this invention, the coating being codeposited froma bath containing preferably from 2 to 5 percent aluminum and from 2 to20 weight percent silicon based on the weight of the sodium bath. Acoating approximately 0.5 mil thick of codeposited aluminum and siliconhas been obtained in one minute from a molten sodium bath maintained ata temperature of 1150 F. and containing 10 percent silicon and 4 percentaluminum. Under similar conditions, coatings of 0.75 mil have beenobtained in 10 minutes and coatings of about 1.5 mils in 30 minutes. Itwill of course be understood that the rate of deposition of a diffusioncoating is not a linear function but depends upon the relationship andinteraction of many factors not fully understood. In actual practice,the rate of diffusion appears to vary logarithmically with time.

Agitation of the molten sodium bath is an important factor that affectsthe time required to form a satisfactory diffusion coating layer of agiven quality and thickness on a base metal. The forming of asatisfactory aluminum diffusion coating presents a particularlydifficult problem because of the apparent formation of a diffusionbarrier at the liquid-liquid interface between the aluminum and thesodium, presumably caused by an oxide film of aluminum. It is believedthat the formation of this thin impermeable alumina film at theinterface partitions the liquid aluminum from the liquid sodium therebypreventing proper solution of the former in the latter and thuspreventing the subsequent mass transfer and reaction with the surface ofthe base metal. However, by continuously disrupting the liquid-liquidinterface, metallic sodium is brought into contact with metallicaluminum permitting solution, transfer and reaction. One technique foreffecting this continuous disruption of the interface is by mechanicalagitation of the reaction chamber and its contents, for example by useof rotating and seesaw type capsules. Alternatively, the capsule orcontainer and its contents may be vibrated, or the contents only or theliquid only vibrated, to cause a wave or ripple action at theliquid-liquid interface. Suitably, mechanical movement of a rake orchain through the liquid may be used to cause interface disruption. Alsofeasible is introduction of an inert gas stream below the liquid-liquidinterface to agitate the interface. To obtain the desired interfacedisruption, the target to be diffusion coated may be maintained inmotion in addition to agitation of the molten sodium bath.

A batch process or a continuous diffusion coating technique may be usedin the process of this invention. Generally a batch process is preferredbecause of the greater degree of control feasible with respect to thedominant parameters of the process. Conveniently, for a batch process acontainer to hold the bath and the specimens to be diffusion coated isfabricated of mild stainless steel tubing of desired diameter cut todesired lengths. An end cap is welded to one end to form a vessel intowhich the bath materials are placed, and another end cap is welded onthe opposite end. Specific capsule configurations may also be used tosupport some of the constituents, to provide a reservoir, or to houseunusually shaped specimens. The specimen to be coated may be placed intothe bath loose, suspended on wire, or encased in a screen envelope.Various types of furnaces may be used to house the capsule, the simplestbeing a static furnace which is temperature controlled to maintain anisothermal environment for the capsule. In other applications, a tubefurnace is used inclined at 30 from the horizontal within which thecapsule is rotated about its longitudial axis at about 15 rpm.Additionally, a rocking motion may be added to the rotating motion. Forthis type of application, a tube furnace is used in which the capsule isrotated at 15 rpm. While the furnace and capsule are both rocked through-30 from the horizontal about a mid-point pivot.

An open vessel bath may be used for both a batch and a continuousprocess. The temperature is maintained by a furnace around the vesselcontaining the bath. A recirculating argon environment is maintainedover the bath solution to prevent any oxidation occurring. Bafiles areinstalled above the open bath to keep the vapor within the reactionvessel and to reduce the heat transfer to the argon atmosphere in a drybox which is used to house the open vessel.

The following examples are offered to illustrate the scope and practiceof this invention in greater detail, and are not intended to beconstrued as limitations thereof.

Example I A stainless steel capsule 1 /2 inches in diameter by 6 incheslong was 'half filled with sodium. About 10 grams of aluminum was added,and the capsule Welded tight under a helium atmosphere in a glove box.The capsule was placed in a rotating jig inside a tubular furnace. Thejig was made to rotate at r.p.m. with its axis of rotation at a 30 anglewith the horizontal.

The capsule was heated while rotating, for 18 hours at 1100 F., and for70 hours at 1500 F. The diffusion layer obtained is .004 inch. The Knoophardness of the diffusion layer (100 gram load) is 417 KHN, while theKnoop hardness of the parent stainless steel is 200 KHN.

Example II The same as Example I, except that aluminum was deposited oncarbon steel by heating aluminum dissolved in sodium at 1500 F. forabout 50-70 hours. A diffusion coating of iron-aluminum was produced.Following this with a diffusion annealing treatment served to diffusethe aluminum further into the steel and produced a highly protectivecoating.

Example III A mild steel specimen (AISI 1010l020) was suspended in astainless steel capsule together with 40 grams of sodium and 10 grams ofaluminum. The capsule was rotated for a period of 43 hours at 1450 F. Acoating 40 mils thick was obtained. This coating is extremely hard, witha Knoop hardness number of 890 (100 gram load).

A similar coating, 80 mils thick was obtained by treating another mildsteel specimen in a bath containing 50 grams sodium and 15 gramsaluminum for 7 hours at 1675 F. X-ray fluorescence analysis of thiscoating showed a major phase of iron-aluminum inter-metallic compound(Fe Al an intermediate phase of chi-molybdenum-chromium-iron (Mo Cr Feand minor phases repeating the intermediate phase and aluminum-iron (AlFe).

Example IV Mild steel specimens were suspended in an open vessel inwhich a recirculating argon environment over the bath solution providedan inert atmosphere. Temperature was maintained by a furnace around thevessel containing the bath. About 10 pounds of sodium filled the vesselto a depth of 5-6 inches, depending upon the operating temperature.Bafiles were installed above the open bath to keep the vapor within thereaction vessel, and to reduce the heat transfer to the argonatmosphere. The bath contained 10 pounds sodium and grams aluminum, theexcess amount of aluminum providing a large reserve pool to insureuniform repeatable processing. The aluminum covered the bottom of theprocessing vessel to a depth to about of an inch and was continuouslyagitated by wiper blades on the bottom of a rotating basket so as tostir the molten aluminum resting in the bottom of the vessel and breakup any oxide film on the aluminum so as to allow pure aluminum to gointo solution readily. Runs in this bath at 1500 F. for 3 /2 to 5 /2hours produced a very heavy diffusion coating on mild steel specimens.Coatings up to 16 mils in thickness were obtained.

Other ferrous base metals such as a low-carbon enameling iron (0.02percent carbon), a titanium-stabilized steel, and various low carbonproprietary steels were similarly coated using this open bathprocessing.

Example V Stainless steel specimens (type 304) were aluminized at atemperature of 1500 F. and 1600 F. for 2 hours and 5 hours in a capsuleusing a rotating-rocking motion. The capsule was rotated atapproximately 15 rpm. while the tube furnace containing the capsule andthe capsule itself were also rocked through i30 from horizontal about amid-point pivot. Diffusion coatings of 1 to 3 mils were obtained. Theamount of aluminum used varied from 0.05 to 0.1 gram aluminum per squareinch of surface coated. The coating was evenly distributed and lightgrey in color. The Knoop hardness of the base metal varied from 178 to252. The case (diffusion coating layer) varied from 420 to 1000 KHN,showing a very marked increase in the surface hardness of the diffusioncoated specimens.

Aluminizing of types 316 and 321 stainless steel produced very similarresults to the diffusion coatings obtained with the type 304 stainlesssteel.

Example VI A high strength austenitic nickel-chromium steel (nominally25% Ni, 15% Cr, 2% Ti, 0.02% Mn, 1.3% Mo, 0.3% V, balance Fe) wasaluminized in the form of small fasteners and bolts and nuts. A thinaluminum diffusion coating with minimum change in thread dimensions wasobtained in runs in a rotating-rocking furnace at 1250 F. for 2 hours.Approximately 5 grams of aluminum was used with 50 grams of sodium. Thespecimens had uniform smooth coatings with case steps of 0.3 to 0.5 mil.

Example VII A nickel base alloy (Hastelloy N, nominal composition weightpercent, C 0.8, Cr 7, Mo 17, Fe 5, Ni balance) was readily aluminized inthe sodium bath. Using an open vessel with aluminum maintained atsaturation in the bath for 3 /2 hours at 1500 F. resulted in thediffusion coating having a smooth appearance, light grey in color with acase of about 1.1 mils thick.

Nickel cobalt alloys were also run in an open bath containing 9 poundssodium and grams aluminum at 1500 F. for 3%. hours, resulting in asmooth even case of 3 mils thick. Hardness test results were: parentmaterial 550, case 860 KHN .(100 gram load).

Example VIII A molybdenum specimen (0.05 inch thick, 6.1 grams) wasaluminized in a capsule containing 50 grams sodium and 15 grams aluminumrun at 1500 F. for 7 hours. A weight gain of 0.49 gram was obtained.

Example IX A columbium (niobium) specimen wasrun at 1675 F. for 7 hoursin a bath of 50 grams sodium and 15 grams 7 aluminum. A case 2.6 milsthick was obtained. The case had a hardness of 846 KHN compared with 113KHN in the parent columbium.

Example X A diffusion-coated case 0.9 mil thick was formed on a tantalumspecimen in a bath containing 50 grams sodium and 6 grams aluminum runat 1450 F. for 7 hours.

Example XI Using a zirconium target in a rotating-rocking capsule, acase 1.2 mils was produced at 1500 F. for hours in a bath containing 50grams sodium and 2 grams aluminum. The aluminized zirconium had a casehardness of 328 KHN compared with the parent material of 153 KHN. In anoxidation test conducted at 1300 F. for 40 hours, the aluminizedzirconium gained 0.13 mg. per square centimeter compared with theuncoated zirconium which gained 4.2 mg. per square centimeter.

Example XII Using a mild steel specimen (AISI -15-1020), a capsule testwas run at 1500 F. for 5 hours in a bath containing 50 grams sodium, 1gram silicon, and 2.5 grams aluminum. A case varying in thickness from 2mils to 4 mils was obtained. This case was found to be more tenacious.than previously siliconized coatings not containing any aluminum.

Example XIII Using a rotating capsule, a solid copper target wasimmersed in an aluminum-containing sodium bath at 1150 F. for 2 hours. Acase 3.5 mils thick was obtained. Oxidation tests at 1350 F. for 50hours produced no visible change on the protected copper, while theas-received copper oxidized quite rapidly.

Example XIV A nickel target was run in a capsule at 1650 F, for 16 hoursin a bath containing 40 grams sodium and 8 grams aluminum. A pleasingappearing 10-mil thick, twophase case was obtained on the nickel base.The hardness of this case was: outer band, 636 KHN; inner band, 846 KHN;parent nickel, 190 KHN.

In an open vessel run in an aluminum-containing sodium bath, a thinner1.6-mil case was produced on the nickel target after immersion for 4hours at 1250 F.

Example XV A columbium alloy (Du Pont D-36, 10% Ti, 5% Zr, balance Nb)was used as a target in a capsule containing 40 grams sodium, 5 gramsaluminum, and 4 grams silicon. The capsule was rotated and rocked at1600 F. for 5 hours. The reaction that occurred involved most of theIO-mil thick target material. In a similar run, where the capsuleadditionally contained 0.5 gram cesium, a sharply defined case 4.5 milsthick was obtained.

Example XVI A tantalum-tungsten alloy (90 Ta-lO W) was immersed in acapsule containing 60 grams sodium and 8.1 grams aluminum and rotatedand rocked at 1450 F. for 5 hours. The case depth obtained was 1.4 mils.

Example XVII 8 Example XVIII Capsule tests were run using mild steelspecimens (1015-1020 AISI) as targets, the bath containing 40 gramssodium and 3 grams aluminum, with the silicon content varying from 3grams to 15 grams. The bath was rotated at 15 r.p.m. while heating to apredetermined temperature of 1400" F. and then held at that temperaturefor 30 minutes to saturate the bath. The capsules were then inverted tosubmerge the specimens in the bath and rotated for 5 minutes. Capsuleswere again inverted and removed from the furnace to cool.

Typical results were as follows: In one run in which the bathcomposition consisted of 3 grams silicon, 3 grams aluminum, and 40 gramssodium, the case obtained was 2.4 mils thick. In another run in whichthe bath contained 3 grams aluminum, 15 grams silicon and 40 gramssodium, a 5-mil thick case was obtained. In another run, to the bathcontaining 3 grams aluminum, 15 grams silicon, and 40 grams sodium, wasadded 3 grams Fe O A 7-mil thick case was obtained.

Spectrographic analysis of a typical 5-mil thick case showed an aluminumcontent of 520%, a silicon content of 5-15%, and iron as a majorconstituent.

Both spectrographic and wet analyses were performed on a case chippedaway from a steel tab which had been immersed in a capsule in which thebath contained 40 grams sodium, 4 grams silicon, 4 grams aluminum, and 2grams FeO, for a period of 15 minutes at 1500 F. Spectrographic analysisof the case showed, weight percent, aluminum 20-50, chromium 5.0,silicon 2-10, and iron as a major constituent. In a more precise wetchemical analysis, the iron content was determined as 38.6%, thealuminum content as 35.5%, and the silicon content as 19.0%.

Example XIX Thin coatings of less than '-1 mil in thickness wereobtained in a series of runs on tabs of mild steel (AISI 1015). Thespecimens were immersed in a presaturated solution and rotated at 15rpm. during the time cycle. The bath composition contained 40 gramssodium, 3 grams a-luminum, and a silicon content varying from 3 grams to15 grams. In two further runs where 40 grams sodium, 15 grams silicon,and 3 grams of aluminum were used, an additional amount of 3 grams ofiron oxide (FeO) was added to one run and 3 grams of nickel oxide addedto another run. The heating cycle was at 1000 F. for times varying from5 minutes to 15 minutes. The case thickness obtained varied from 0.3 to0.8 mil.

Example XX The resistance of coated and uncoated mild steel specimenswas evaluated in a highly corrosive atmosphere simulating thatencountered in the exhaust stream of an automotive mufiier. Thespecimens were suspended for 20 hours above a bath consisting of adilute solution of hydrobromic and sulfuric acids maintained at 180 F.The specimens were then removed and dried for 4 hours at 250 F. tocomplete one cycle.

After five cycles, a sample of uncoated 1015 mild steel was very badlyattacked. A mild steel specimen which had been immersed in analuminum-containing sodium bath at 1600 F. for 5 hours showed a weightgain (mg. per square centimeter) of but 0.56. Specimens which had beenimmersed in a solution of sodium-aluminumsilicon at 1250 F. for 5minutes showed an average weight loss (mg. per square centimeter) of0.08. Another specimen which had been immersed in asodiumaluminum-silicon bath at 1000 F. for 5 minutes showed a weightloss after 5 cycles of but 0.007 mg. per square centimeter.

Thus, while the aluminum-coated mild steel sample was markedly superiorto the untreated mild steel specimen, those specimens which had beenco-deposited with aluminum and silicon showed superior resistance tocorrosion,

compared with only the aluminum-coated specimen, by factors varying from2- to 20-fold.

It will of course be understood that many variations are possible in thepractice of this invention, depending upon the coating thicknessdesired, the base metal used, and the particular aluminum-containingdiffusion coating desired, and these variants are therefore consideredto lie within the scope of this invention. Accordingly, the scope ofthis invention should be determined in accordance with the objectsthereof and the appended claims.

I claim:

1. The method of providing a diffusion coating containing aluminum on abase metal selected from the class consisting of groups.IVB, V-B, andthe first triad of group VIII of the periodic table which comprisesplacing the base metal in a molten sodium bath maintained under an inertenvironment and containing aluminum dissolved therein and maintainingsaid base metal in said bath until a diflusion coating containingaluminum is obtained on said base metal.

2. The method of providing an aluminum diffusion coating on a ferrousmetal which comprises providing a bath of molten sodium under an inertenvironment, dissolving aluminum therein, and placing said ferrous metalin said molten bath until a diffusion layer of aluminum is obtained onsaid ferrous metal.

3. The method of claim 2 wherein said bath is maintained at atemperature between 1000 and about 1500" F. at ambient atmosphericpressure.

4. The method of claim 3 wherein a pool of molten aluminum is maintainedin said bath while coating said ferrous metal.

5. The method of providing an aluminum coating on niobium whichcomprises providing a molten sodium bath maintained at a temperaturebetween 1000 and about 1500 F. under an inert gas atmosphere, dissolvingaluminum in said sodium, placing said niobium in said sodium bath, andmaintaining said niobium in said sodium bath until a diffusion coatingof aluminum on said niobium is obtained.

6. The method of providing a diifusion coating of aluminum on molybdenumwhich comprises providing a molten sodium bath under an inertenvironment, dissolving aluminum in said bath, and maintaining saidmolybdenum in said bath until said diffusion coating of aluminum isobtained thereon.

7. The method of providing a diffusion coating on a base metal whichcomprises dissolving aluminum and silicon in a molten sodium bathmaintained under an inert environment, placing said base metal in saidbath and maintaining said base metal in said bath until a diffusioncoating containing aluminum and silicon is obtained on said base metal.

8. The method of claim 7 wherein said bath is maintained at atemperature between 1000 and about 1500 F. at ambient atmosphericpressure.

9. The method of claim 8 wherein a .pool of molten aluminum ismaintained in said bath while coating said base metal.

10. The method of providing a diffusion coating on a ferrous metal whichcomprises dissolving aluminum and silicon in a molten sodium bathmaintained under an inert environment, placing said ferrous metal insaid bath maintained at a temperature between 1000 and about 1500 F. andmaintaining said ferrous metal in said bath until a diffusion coatingcontaining aluminum and silicon is obtained on said ferrous metal.

11. A method of providing a codeposited diffusion coating of aluminumand silicon on a ferrous base metal which comprises providing a moltensodium bath containing from 2 to 5% aluminum, and from 2 to 20% silicon,by weight of the sodium, said bath being maintained under an inertenvironment, placing said ferrous metal in said bath, and maintainingsaid ferrous metal in said bath maintained at a temperature between 1000and about 1500 F. for a period of time varying from 1 minute to 5 hours,until a difiusion coating of desired thickness containing aluminum andsilicon is obtained on said ferrous metal.

12. A bath for providing a codeposited aluminumsilicon diffusion coatingon a ferrous base metal which comprises a molten sodium bath containing,by weight of said sodium, from 2 to 20% silicon and from 2 to 5%aluminum.

13. The bath of claim 12 in which excess aluminum is present as a moltenpool in said bath.

References Cited by the Examiner UNITED STATES PATENTS 2,351,798 6/ 1944Alexander 117-22 2,774,868 12/1956 Hodge 117-114 2,848,352 8/1958 Nolandet al 117-114 X 2,894,856 7/1959 Schwendemann et al. 117-114 2,914,419 11/ 1959 Oganowski 1117-114 X 2,929,740 3/ 1960 Logan 117-114 2,991,1977/1961 Sandoz 117-114 X 3,073,720 1/1963 Mets 117-131 3,085,028 4/1963Logan 117-114 3,086,886 4/1963 Kiefier et al 117-114 OTHER REFERENCESHansen: Constitution of Binary Alloys, Metallurgy and MetallurgicalEngineering Series; McGraw-Hill Book Co., 2nd Ed. (1958), page 91.

RICHARD D. NEVIUS, Primary Examiner.

UNITED STATES PATENT OFFiCE CERTIFICATE OF CORRECTION Patent N06 3 ,220,876 November 30 1965 Roger D. Moeller It is hereby certified that errorappears in the above numbered patent requiring correction and that thesaid Letters Patent should read as corrected below.

Column 10 line 39, for "2,774 ,868" read 2 ,774 686 rmumhwmh Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNE Commissioner ofPatent

1. THE METHOD OF PROVIDING A DIFFUSION COATING CONTAINING ALUMINUM ON ABASE SELECTED FROM THE CLASS CONSISTING OF GROUPS IV-B, V-B, AND THEFIRST TRIAD OF GROUP VIII OF THE PERIODIC TABLE WHICH COMPRISES PLACINGTHE BASE METAL IN A MOLTEN SODIUM BATH MAINTAINED UNDER AN INERTEVIRONMENT AND CONTAINING A LUMIUM DISSOLVED THEREIN AND MAINTAININGSAID BASE METAL IN SAID BATH UNTIL A DIFLUSION COATING CONTAININGALUMINUM IS OBTAINED ON SAID BASE METAL.